1. Field of the Invention
The invention relates to a thermal head and a thermal printer which form an image on a recording medium by pressing a protruding portion on which heating elements are arranged on the recording medium while driving heating elements to be heated. Particularly, the invention relates to a technique for improving heat resistance and breaking strength of the thermal head.
2. Description of the Related Art
A thermal printer including a thermal head having heating resistive elements (heating elements) arranged on a protruding portion and a platen roller provided so as to face the thermal head is conventionally known. Such a thermal printer forms an image by pressing the protruding portion of the thermal head on the printing paper (recording medium) carried on the platen roller. The protruding portion and the printing paper are pressed by moving the thermal head or the platen roller.
The thermal printer has a sublimation method, a heat sensitive method and the like as an image forming method. In any method, power is selectively applied to the heating resistive elements of the thermal head according to the tone level, and the image is formed by using thermal energy generated at that time. For example, in the case of a sublimation-type thermal printer, when the protruding portion of the thermal head is pressed on the printing paper through an ink ribbon and the heating resistive elements are driven to be heated, ink on the ink ribbon is sublimed on the printing paper in proportion to thermal energy of the heating resistive elements to perform printing.
As described above, the thermal head heats the heating resistive elements for performing printing, and most of the heat generated from the heating resistive elements at the time of printing is transmitted in the opposite direction of the printing paper and released. Therefore, in order to print at high speed, it is necessary to heat the heating resistive elements at high temperature immediately, however, there arises a problem that power consumption increases. Since it is necessary to increase a printing speed while saving power particularly in a thermal printer for home use, it is desirable to improve thermal efficiency of the thermal head to reduce power consumption.
A technique of improving thermal efficiency and response of the thermal head in order to reduce power consumption of the thermal printer as well as to print high quality images or characters at high speed is known. Specifically, in the technique, a gap portion is formed in a glass substrate in which heating resistive elements are arranged, and an air layer in the gap portion makes heat generated from the heating resistive elements difficult to be released in the direction of the glass substrate to improve thermal efficiency as well as the gap portion reduces the heat accumulation amount of the glass substrate to improve response (For example, refer to JP-A-2007-245675 (Patent Document 1)).
FIG. 9 is a longitudinal sectional view showing a thermal head 110 in related art, which is disclosed in the Patent Document 1.
As shown in FIG. 9, in the thermal head 110, a heating resistive element 112, a power supply electrode 113a and a drive electrode 113b for heating the heating resistive element 112 and a protective film 114 for protecting the heating resistive element 112, the power supply electrode 113a and the drive electrode 113b are sequentially stacked on a glass substrate 111 to form a head body portion, on which a protruding portion 111a having an approximately arc-shape in vertical section is formed. A portion between the power supply electrode 113a and the drive electrode 113b, in which the heating resistive element 112 is exposed, is a heating portion 112a which actually generates thermal energy.
The heating portion 112a having a rectangular shape of a length L1 is provided on the protruding portion 111a so that the heating portion 112a can be pressed on the ink ribbon and the printing paper. In the glass substrate 111 on which the protruding portion 111a is formed, a concave gap portion 111b having a width W2 which faces the protruding portion 111a is also formed. The width W2 of the gap portion 111b is formed to be larger than the length L1 of the heating portion 112a (gap portion width W2>heating portion length L1) and the gap portion 111b is filled with air. Furthermore, a heatsink 115 adheres to the bottom of the glass substrate 111 by an adhesive 116 so as to close an opening surface of the gap portion 111b. 
In the above thermal head 110, thermal conductivity in the gap portion 111b is low due to characteristics of air having lower thermal conductivity than glass forming the glass substrate 111. That is, since the width W2 of the gap portion 111b is larger (gap portion width W2>heating portion length L1), the amount of air in the gap portion 111b becomes large, and heat release from the heating portion 112a provided on the protruding portion 111a of the glass substrate 111 to the direction of the glass substrate 111 is suppressed. Therefore, thermal energy transmitted in the direction of the ink ribbon pressed by the protruding portion 111a is increased. As a result, power consumption which is necessary for increasing a temperature of ink on the ink ribbon to the sublimation temperature of the ink is reduced, which improve thermal efficiency of the thermal head 110.
Additionally, the thickness of the protruding portion 111a of the glass substrate 111 is reduced by the gap portion 111b and the heat accumulation amount of the glass substrate 111 is reduced, therefore, thermal energy accumulated in the glass substrate 111 can be released in a short time. As a result, when ink on the ink ribbon is not sublimed (when the heating portion 112a is not heated), the temperature of the heating portion 112a decreases immediately, which improves response of the thermal head 110.